June 2019, ScientificAmerican.com 35cle physics called the Standard Model, which describes
the known forces of the universe (apart from gravity).
Just as the electromagnetic force between electrically
charged particles is carried by photons, or particles of
light, QCD tells us that the strong force—the force hold-
ing nucleons together—is carried by gluons. The “charge”
involved in the strong force is called “color” (hence
“chromodynamics”). Quarks carry color charge and
interact with one another by exchanging gluons. But
unlike electromagnetism, where photons themselves
have no electric charge, gluons carry color. Therefore,
gluons interact with other gluons by exchanging more
gluons. This wrinkle has profound implications. The
feedback loop of interactions is why QCD is often too
complicated to compute.
QCD also differs from more familiar theories be -
cause the strong force becomes weaker the closer to -
gether quarks get. (In electromagnetism, the opposite
is true, and the force gets weaker as charged particles
move farther apart.) At short enough distances within
the nucleon, the quarks feel so little force they behave
as if they are free. The discovery of this strange conse-
quence of QCD won physicists David Gross, H. David
Politzer and Frank Wilczek the 2004 Nobel Prize in
Physics. When quarks move away from one another,
the force between them grows rapidly and becomes so
strong that quarks end up “confined” within the nucle-
on—that is why you will never find a quark or a gluon
alone outside a proton or neutron. Scientists can calcu-
late QCD interactions as long as the quarks are close
together and interact weakly with one another; when
they are farther apart, however—at distances close to
the radius of the proton—the force becomes too strong,
and the theory becomes too complex to be useful.
To understand the quantum realm of the strong
force further, we need more information. Our mastery
of the atomic realm, for example, did not come only
from our understanding of atoms and their interac-
tions—it came from our grasp of the emergent phe-
nomena that arise on top of these fundamental build-
ing blocks. It was not possible to construct molecular
biology from our knowledge of its foundations—atoms
and electromagnetism. The eureka moment came
when researchers discovered the double-helix structure
of DNA. What we need to make progress in the quark-
gluon world is to look inside the nucleus.“ SEEING” ATOMS
in the first part of the 20th century physicists discov-
ered how to “see” atoms through a process called x-ray
diffraction. By shining a beam of x-rays at a sample and
studying the interference pattern that results when they
pass through the material, scientists could see its atomic
crystal structure. The reason this technology works is
that the wavelength of an x-ray is similar to the size of an
atom, giving us the ability to probe the atomic distance
scale of nanometers (10-9 meter). In the same way, phys-
icists first “saw” quarks 50 years ago in an experiment
that collided electrons and protons in a process called
deep inelastic scattering, or DIS.
In this method, an electron bounces off a proton
(or neutron or nucleus) and exchanges a virtual pho-
ton with it. The virtual photon is not exactly real—it
pops in and out of existence quickly as a consequenceBLUE DIPOLE
MAGNETS help
to steer elec
tron beams as
they accelerate
around the
CEBAF loop.